Abstract

An extensive survey of the Mid-Atlantic Ridge from 47° to 51° N. has recovered a large quantity of detailed bathymetric data from the North Atlantic sea floor. Supplemented by seismic reflection and magnetic profiles, these data reveal some surprising characteristics of sea-floor spreading on the Mid-Atlantic Ridge. Closely spaced transform faults, generated about 60 m.y. B.P., extend to crust of about 20 m.y. B.P. age. Significantly, however, demonstrable transform faults are nearly absent on ocean crust younger than about 10 to 20 m.y. B.P. Possibly in response to recent increased spreading rates or increased asthenosphere flow below the spreading axis, the ridge axis has replaced the transform pattern within about the last 20 m.y. B.P. with alternating normal and oblique spreading axes. The normal spreading segments are generating a basement level 0.5 to 1 km higher than that produced by the oblique segments, probably because of greater viscous head losses for magma rising within oblique rifts. The late Tertiary spreading pattern of normal and oblique rift segments has not been stationary with respect to the transform direction but has migrated southward at 1 or 2 mm per yr, possibly a measure of slow southward subaxial motion in the asthenosphere. A problem with this hypothesis is that the rift-valley floor and the regional bathymetry seem to be shoaling very gradually (1:1,000) toward the Azores, which suggests a northward-directed pressure gradient in the asthenosphere. Although the top of the oceanic basement in the northeastern Atlantic deepens with increasing age, there are large depth anomalies with respect to crustal subsidence in the eastern Pacific. Crust of ages 60 to 50 and 10 to 0 m.y. B.P. is elevated about 1 km above coeval Pacific crust, with lesser anomalies occurring elsewhere. Because the highest basement levels seem to correlate with probable times of most intense plume convection, it is inferred that a lithosphere of lower density or thicker crust is formed at such times.

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